Thin printing paper webs containing mechanical pulp are conventionally coated on subsequent coating stations equipped with short-dwell coaters as the coater units. A combination of two subsequent coater stations is necessary as low base paper weight and high content of ground wood make single-run coating on both sides impossible. Two-side coating with conventional methods would excessively wet the web and thus impair its runnability. Furthermore, the measurement of coat weight in a two-side coating operation is difficult.
Despite their multiple benefits, short-dwell coaters also have several drawbacks. Air entrapped in the coat paste easily causes mottling. Because of the small linear application pressure and short application distance, wetting of the base web and subsequent fiber swelling occurs even after the web has passed the doctor blade, thereby impairing the smoothness of the coat being applied. As a rule, doctor blade coating methods become critical with thin webs and particularly with light coat weights.
Paper grades containing a high proportion of mechanical pulp and a high percentage of coat fillers such as, e.g., SC paper (wood containing high filler content super-calendared printing paper) make doctor blade coating impossible because of the fragility of the base paper web.
Web defects leading to brittleness result in low production yield and inferior runnability.
Base paper grades containing recycled fiber have posed unexpected problems in doctor blade coaters, specifically, the coat is easily marked during coating by streaks caused by defective doctor blades.
The darker color of recycled-fiber containing base paper grades make the opacifying power of the coat mix more critical. Because of the way in which doctor blade coating is accomplished, a smooth coat is formed, not a coat of uniform coat weight. As the base paper is not necessarily smooth, the opacifying power obtained in doctor blade coating is insufficient, resulting in mottling of the coated web.